1. Field of the Invention
The present invention relates to an internal combustion engine, and more particularly to an internal combustion engine having an air-fuel ratio sensor disposed on an exhaust passage.
2. Description of the Related Art
In general internal combustion engines, the air-fuel ratio of an exhaust gas is feedback-controlled by detecting the air-fuel ratio with an air-fuel ratio sensor installed on an exhaust passage and making adjustments such that the detected air-fuel ratio coincides with a predetermined target air-fuel ratio. When the air-fuel ratio sensor (specifically a sensor element) is at low temperatures and hence not activated, for example immediately after starting up the internal combustion engine, the air-fuel ratio sensor cannot detect the air-fuel ratio. Thus, in order to activate the air-fuel ratio sensor more quickly, the air-fuel ratio sensor is provided with a heater for heating the air-fuel ratio sensor while the heater is energized. This allows the air-fuel ratio sensor to start detecting the air-fuel ratio quickly after the start up of the internal combustion engine, thus allowing the feedback control of the air-fuel ratio to start quickly, and emission (specifically cold emission) after engine startup to be reduced.
When the exhaust passage is cold, for example immediately after engine startup in a cold state, the exhaust gas is cooled and water vapor contained in the exhaust gas is condensed to produce condensed water. Under such conditions, if the condensed water adheres to the sensor element while being heated, the sensor element may be cracked by “thermal shock.” Therefore, the sensor element cannot be heated even when it is desired to do so, causing unfavorable delay of the start of the feedback control of the air-fuel ratio and difficulties in improving emission immediately after engine startup.
Various proposals have been made so far to prevent condensed water from adhering to the sensor element of the air-fuel ratio sensor, and the sensor element from being cracked. For example, the air-fuel ratio sensor disclosed in JP-A-9-222416 has inner and outer covers for covering a sensor element. The covers are formed with openings for introducing an exhaust gas to the sensor element. The openings of the covers are not in alignment with each other in order to lengthen the exhaust gas flow passage through the openings, thus making it difficult for condensed water to reach the sensor element.
With the structure of the air-fuel ratio sensor disclosed in JP-A-9-222416, however, the exhaust gas also cannot easily reach the sensor element, which may delay the detection of the air-fuel ratio, and may worsen the air-fuel ratio detection response as well as the precision of the air-fuel ratio feedback control.
Meanwhile, turbochargers are known as means for increasing the output power of internal combustion engines. In an internal combustion engine provided with a turbocharger, a turbine disposed on an exhaust passage increases the heat capacity of the exhaust passage. Therefore, it takes a relatively long time to warm the exhaust passage, and condensed water is produced in an increased amount and/or for a longer time. With an air-fuel ratio sensor installed downstream of the turbine, it is more likely that the air-fuel ratio sensor will be wetted with condensed water, and that the sensor element will be cracked.